At that stage I would just kind of give a tired smile and say I was feeling pregnant. Generally the people asking me genuinely wanted to know so I wouldn't lie but like you I tried not to make it too complainy.

if the people asking have been pg or their spouses/partners have, you could smile & answer with a question for them, "What do you recall about pg at 15wks?" or "I'm looking forward to my favourite trimester. What was yours?"
Something to get them talking & off on a tangent so you can rest?

I felt awful for exactly 16 weeks this time so I know exactly how you feel. People constantly want to ask and hey really want to know how you are feeling so just tell them. "I'm sick about 90% of the time, but I think it's getting better" was all I would say for that entire time.